Lasers are widely used in the manufacture of advanced printed circuit boards (PCBs). A particularly well known example is the drilling of blind contact holes, so called micro-vias, in multi layer PCBs. In this case ultra violet (UV) lasers are often used to drill through a top copper layer and an underlying dielectric layer to allow contact to, be made to a lower copper layer. In some cases, the cost effectiveness of this process is improved by using two different laser processes to remove the two different materials. A UV diode pumped solid state (DPSS) laser is usually used to drill the holes in the top copper layer to expose the lower dielectric layer and in a separate process a CO2 laser is used to remove the dielectric material exposed below each hole. Such a two stage laser process can have many economic advantages as each process can be optimized separately. In such a case, however, because of the very different optical requirements, two physically separated optical systems are used and the substrate has to be moved, between the two systems to enable the second process to be performed.
Recently a new type of high density multi-layer circuit board manufacturing technology has been proposed. US2005/0041398A1 and publication “Unveiling the next generation in substrate technology”, Huemoeller et al, 2006 Pacific Micro-electronics Symposium describe the concept of “laser-embedded circuit, technology”. In this new technology lasers are used to directly ablate fine grooves, larger area pads and also blind contact holes in organic dielectric substrates. The grooves connect to the pads and contact holes so that after laser processing and subsequent metal plating a complex pattern of fine conductors and pads embedded in the top surface of the dielectric layer is formed together with deeper contact holes connecting to a lower metal layer. Up to now pulsed UV lasers have been used to form the grooves, pads and contact holes in a single process using either direct write or mask imaging methods.
The direct write approach generally uses a beam scanner to move a focussed UV laser beam over the substrate surface to scribe the grooves and also create the pad and contact hole structures. This direct write approach uses the highly focusable beam from UV lasers with high beam quality and hence is very well suited to the fine groove scribing process. It is also able to deal well with the different depth requirements associated with pad and contact hole structures. By this method, grooves, pads and contact holes of different depth can be readily formed. However, because of the limited laser power available from highly focusable UV laser, this direct write process is slow when it comes to removing the more substantial volume of material associated with the larger area pads and contact holes. This direct write method also has difficulty maintaining constant depth at the intersections between grooves and pads.
The mask imaging approach uses an excimer laser to illuminate a mask containing the full detail of the circuit design. An image of the mask is projected onto the substrate and the mask and substrate are moved together to allow the full area of the circuit to be reproduced on the substrate. Since the whole area of the mask is scanned during the image transfer process, this approach is insensitive to the area of the structures to be created and hence is well suited to creating both the fine grooves and also the larger area pads. It is also excellent at maintaining depth constancy at the intersections between grooves and pads. However, except in the case where the circuitry is extremely dense, this mask imaging is approach is significantly more costly than the direct write approach since the purchase and operating costs of excimer lasers are both very high. Mask imaging is also very inflexible in that a new mask needs to be manufactured each time a new circuit design is required. In addition, excimer laser mask projection is not well suited to creating structures of different depth as required especially for the contact holes.
This later limitation is overcome in the invention described in publication US 2008/0145567 A1. In this case, an excimer laser scanning mask projection system is used to simultaneously form grooves and pads to the same depth in the insulating layer and, in a separate process, using a second laser which is delivered by a separate beam delivery system, the deeper contact holes penetrating to an underlying metal layer are formed. This two step process is an effective way of dealing with the varying depth structure requirements but still suffers from the high cost and poor flexibility associated with the use of masks and excimer lasers.
Hence, it can be seen that the existing process methods for making advanced circuits based on this “laser-embedded circuit technology” have serious disadvantages. There remains a need to be able to use laser processes that are separately optimized for creating the different size and depth structures required in a very flexible way, to improve the process rate and reduce the cost. The present invention aims to provide a multistage stage process that avoids the use of large area masks and costly excimer lasers yet addresses these needs.